1. Field of the Invention
The present invention relates to analog front ends for high speed serial communication and more particularly to analog transmission envelope detectors operable at the high frequencies of USB2.0.
2. Description of the Related Art
With increases in operating frequencies of central processing units (CPUs) of personal computers (PCs) on the order of several GHz, there have been proposed various high-frequency interfacing systems. For example, there are peripheral component interconnection (PCI) bus systems and IEEE 1284, as kinds of parallel interfacing schemes. Parallel interfacing schemes offer sufficient bandwidth, but there is a disadvantage of too large a data width which increases the number of wires required and the weight and size of cables. Further, parallel interfacing schemes require additional signal lines for control signals. And, if the number of signal lines increases in small apparatuses such as mobile phones, electro-magnetic interference (EMI) increases as well as degradation of power efficiency. And, the increased number of signal lines occupies more space for the signal lines, making it difficult to miniaturize the small apparatuses. Therefore, consumer product manufacturers are actively studying to change data transmission methods from parallel schemes to serial schemes.
Mobile display digital interface (MDDI) is an interface scheme assisting serial communication between mobile-phone modems and liquid crystal display (LCD) units. The MDDI is capable of reducing the number of signal lines to about one-tenth ( 1/10) of the parallel interface scheme and transceiving data with low power, which provides high power efficiency in use.
High speed serial interface schemes include IEEE1394 and universal serial bus (USB). The IEEE1394 originally provided a wide bandwidth of 400 Mbps and is widely used in audio-visual (AV) products such as camcorders, digital cameras and in modems. And, more recently with the advent of USB2.0, the IEEE1394 increased its bandwidth up to 480 Mbps at maximum, which enables real-time transmission of multimedia data. Constructing a system with USB scheme is simpler than with IEEE1394. Since the USB is able to select bandwidths in accordance with operating speeds of peripheral devices, it is highly advantageous in cost and efficiency.
FIG. 1 is a block diagram of a conventional analog front end including a conventional transmission envelope detector for USB2.0. The configuration of the analog front end for the USB2.0 is specified in “Universal Serial Bus Specification Revision 2.9”, published by USB-Implementers Forum Inc. on Apr. 17, 2000. The analog front end is a low level analog circuit conducting physical connections through D+/D− (DP/DM) signal lines. The USB2.0 is further comprised of circuits to perform high-frequency communication, such as a transmission envelope detector 10, a high-speed (HS) differential data receiver 20, and a HS current driver 30.
High-speed (or high-frequency) data transmission is carried out by flowing current through one of two transmission lines. For instance, the D+ line flows a current to transmit a data bit of a logical “high” (e.g., one) and the D− line flows a current to transmit a data bit of logical “low” (e.g., zero). For that purpose, the HS current driver 30 switches a current of 17.78 mA toward the D+ or D− line.
The HS differential data receiver 20 is used for receiving data at a high speed. The quiescent state of a high speed (HS) link is for the D+ and D− lines to be balanced near ground with the differential receivers listening for a “Start of Packet”. The Transmission Envelope Detector is invoked to prevent spurious signals (e.g., noise, crosstalk, or oscillation) from triggering the “Start of Packet” detection process (to “squelch” the receiver).
The transmission envelope detector is used to disable or “squelch” the high speed (HS) receiver when the amplitude of the differential signal falls below the minimum required level for data reception, preventing noise from propagating through the receive logic. The conventional transmission envelope detector 10 compares data that is received through the HS differential data receiver with a reference voltage of a predetermined magnitude and then determines whether the received data is valid or noise. For example, if the received data is smaller than the reference voltage, the envelope detector 10 determines that the received data is noise and accordingly disables peripheral circuits. If the received data is larger than the reference voltage, the envelope detector 10 determines the received data is valid data and enables the peripheral circuits accordingly. The peripheral circuits maintain the power in a low level state (or a power-off state) or change the power condition to an operation mode, in response to the result of the validity determination.
The reference voltage used in the comparison made by the envelope detector 10 is predesignated as 0.125V by the USB2.0 specification. In general, the reference voltage is supplied from an external source. However, when using the external reference voltage, it is difficult to deal with common-mode (CM) voltage variation because the reference voltage level is independent and fixed. And it is complicated to construct the circuit due to additional structures of a first differential amplifier to amplify a difference between a reference voltage and an input signal received from the D+ line, and a second differential amplifier to amplify a difference between the reference voltage and an input signal received from the D− line. Without accepting the reference voltage from the external source, it is necessary to determine the CM value by using an additional circuit (e.g., a CM detection circuit). Thus, settling a CM voltage takes a long time, and constructing the circuit is complicated as well.
The validity determination for a received signal in the transmission envelope detector 10 is used for quickly deciding the operational mode of other circuits. In the high speed interface environment, such as MDDI and USB2.0, the envelope detector 10 is required to determine the validity of the received signal within the range of several nanoseconds or several bits. In particular, considering that the interface schemes of MDDI and USB2.0 are HS interface systems applicable to portable apparatuses such as mobile phones, the envelope detector 10 needs to be small so as to enhance spatial efficiency therein. The transmission envelope (data-validity) detector 10 needs to be capable of dealing with variations in the common mode (CM) voltage. And, the transmission envelope (data-validity) detector 10 needs to be capable of functioning even in a low voltage environment considering that power supply voltage levels of portable apparatus are being lowered.